The present invention relates to printers, and more particularly, to thermal ink jet print cartridges.
Thermal ink jet print cartridges are extensively used in printers attached to personal computers. Such print cartridges are also sometimes referred to as pens. They provide good quality print and fast dry time on a variety of media, including common papers. They enable non-contact printing of both color and black and white text, graphics, and images, eliminating printer failures due to friction wear and foreign body interference. Their self-contained design and direct printer interconnect allows fast, simple replacement, while avoiding the necessity for ribbons, pumps etc. Thermal ink jet print cartridges are small, and virtually silent in operation. They have relatively low power consumption and EMI emissions.
A conventional ink jet print cartridge has an injection molded plastic outer rectangular housing with suitable projections and notches for precision registration in a reciprocating carriage of a printer. The plastic housing includes an ink reservoir. A nozzle plate on the outside of the plastic housing has a plurality of nozzle orifices. Underneath each nozzle orifice is a firing chamber (ink ejection cavity) commonly fed with ink from a plenum connected to the reservoir. Ink is expelled through each nozzle utilizing a corresponding resistor element which rapidly heats a minute quantity of ink in response to an energizing signal controlled by a microprocessor in the printer. In effect the minute quantity of ink is boiled and spit out of an orifice to form a dot on the print media. The vapor bubble grows rapidly and gives momentum to the ink above the bubble which in turn is propelled through the orifice in the nozzle plate. Ink rapidly refills the firing chamber from the plenum via capillary action.
Techniques have been developed for inexpensively manufacturing the aforementioned thermal printhead structure using well known integrated circuit fabrication techniques. A thin film substrate provides the resistor-conductor structure for thermally exciting the ink to eject it through the nozzles in the nozzle plate. The printhead resistor-conductor structure is typically fabricated on a glass substrate using standard thin film deposition and etching techniques. A dielectric material such as sputtered silicon dioxide is first deposited first onto the glass substrate as a barrier film to prevent leaching of impurities from the glass into the resistor and conductor films. The resistor film is tantalum-aluminum and is sputter deposited using a magnetron. Aluminum doped with a small percentage of copper is next deposited by magnetron sputtering to form the conductor film. The resistor-conductor films are photolithographically patterned to form a column of resistors connected by a common conductor on one end and terminated by an array of individual aluminum electrical contact pads on their other ends. The resistors are covered with ink-resistant passivation films such as silicon carbide over silicon nitride. The electrical contact pads are typically formed of nickel and make contact through the passivation layers with the underlying aluminum-copper conductor film layer. When the print cartridge is installed in the printer, the electrical contact pads register with a corresponding array of contact pads in the printer carriage which are in turn connected to a circuit board in the printer through a flexible ribbon cable. To improve electrical contact pad reliability, the electrical contact pads are coated with gold film.
A nozzle plate made of electroformed nickel with a plurality of individual nozzle orifices is attached to the thin film structure such that each orifice is aligned with respect to the resistors. A capillary ejection cavity exits between each nozzle orifice and resistor. To print a dot, the selected resistor is energized by a suitable electrical pulse and rapidly heated to several hundred degrees C. in a few microseconds. The ink-vapor bubble formed adjacent to the resistor propels an ink droplet out of the nozzle orifice to form a dot on the adjacent paper or other print media. After the electrical pulse terminates, the vapor bubble collapses, subjecting the thin film substrate passivation to severe hydraulic forces. Thus, during operation of the printhead, the passivation experiences severe electrical, thermal, mechanical and chemical stresses.
The thin film structure of a conventional thermal ink jet printhead is inherently subject to defects during fabrication. Any type of defect that might allow ink to reach the thin film metalization is normally fatal to the proper operation of the printhead. Such defects include pinholes intrinsic to the passivation, particulate inclusions and minute crevices (micro-cracks) along conductor edges. Optimization of the deposition processes can adequately address pinholes and particulate inclusions. However, crevices adjacent the edges of the electrical contact pads have been particularly problematic. Any abrupt slope discontinuity in the passivation at this edge is likely to cause a failure. To avoid this, the edges of the through holes in the passivation layers into which the electrical contact pads extend are beveled to improve the subsequent step coverage. However this beveling is difficult to control and is very sensitive to surface quality, materials, and process variations.
The perimeters of the electrical contact pads are particularly susceptible to corrosion because the normal protective films (passivation) must be etched away at the location of the electrical contact pads in order to achieve electrical connection with the corresponding internal aluminum-copper traces. The etched areas are plated up with nickel to form durable contacts that are used to physically mate with corresponding contact pads in the printer carriage. Unfortunately, the etching process followed by the plating process does not ensure a hermetic seal. Even the tiniest sealing flaw allows moisture and oxygen to penetrate the corrosion susceptible films via minute crevices. Once oxydation or other corrosion initiates it can rapidly propagate (filiform corrosion) due to high humidity in either the test environment or the actual use environment. This corrosion can cause serious printhead operation failures. Efforts to provide a commercially viable solution that will prevent corrosion in the electrical contact pads of thermal ink jet print cartridges have heretofore not met with success despite the fact that this problem has existed since the commercial introduction of such cartridges approximately two decades ago.